Q: hi mike, i'm thinking of doing hiit every time after my gym session, how long should my hiit last?
A: HIIT is perfectly fine 2-3 times a week, up to 15 minutes per session. More than that would probably be overdoing it for most people, especially if you also do weight training for legs. Unless you are in a very good shape already, even a few minutes of HIIT should be very challenging, so you may want to start with just a few minutes of HIIT per session and add a minute per week till you build up to 15 minutes per session.